When designing with pneumatic cylinders, it is often desirable to calculate the standard cubic feet per minute (SCFM) of air required to operate the cylinder. This article will explain how to do that calculation.
Let's assume, for example, that you are using a double acting (reciprocating) air cylinder with a 2-inch bore and a 12-inch stroke. Let's further assume that it reciprocates at 10 cycles per minute using 90-P.S.I.G. of air. If you want to know what the required scfm flow-rate of the air to the cylinder is, use the following equation:
Q2 = P2 x Q1 / P1
Where;
Q2 equals the quantity of compressed air required to operate the cylinder based on the volume of the cylinder bore for a given stroke, forward and return.
P1 equals the atmospheric pressure, which is 14.7 pounds per square inch.
P2 equals the gauge pressure of the compressed air being used to activate the cylinder.
Q1 = the piston area * (2 * stroke) * the number of cycles per minute / 1728.
Q2 equals the volume of uncompressed air required.
The area for a 2-inch bore = Pi/4 * D^2 = 3.14-in^2
12-inch stroke, Pressure, P2 = 90-Psig
Cycles = 10 cycles /min
Dividing by 1728 converts cubic inches to cubic feet.
Q1 = Piston area * (2 * stroke) * cycles/minute / 1728
Q1 = 3.14-in2 x (2 x 12) x 10 / 1728 = 753.98 / 1728 = .4363-ft3/ min (cubic feet per minute)
Q2 = P2 x Q1 / P1 = 104.7 x .4363 / 14.7 = 3.11 scfm (standard cubic feet per minute)
So, your compressor and valve must be able to supply 3.11 standard cubic feet per minute of air to operate your cylinder for this design.
When doing these calculations, disregard the piston rod cross-sectional area, and assume that the temperature remains constant. The value "P2" equals the gauge pressure plus the atmospheric pressure, which is 14.7 p.s.i.
The photo is courtesy of Parker Automation Products
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